The number of people using a wheelchair is estimated at 2.2 million
in the United States, 750,000 in the United Kingdom, and 152,400 in the
Netherlands [1]. These individuals spend a large part of their life in
their wheelchair, so their quality of life depends highly on the quality
and comfort of the wheelchair. A wheelchair vibrates while traveling
over obstacles and uneven surfaces, resulting in whole-body vibration
(WBV) of the person sitting in the wheelchair. WBV can result in
decreased comfort, interference with activities, impaired health, pain,
and motion sickness [2]. According to clinicians from the GF Strong
Rehabilitation Centre in Vancouver (British Columbia, Canada), people
with spinal cord injury (SCI) have reported that rough surfaces and
obstacles, such as bumps in sidewalks or rumble carpets, illicit spasms.
However, in the literature, no research has been conducted to support
these reports.

Spasticity and neuropathic pain can result after an SCI. Spasticity
is defined as "a velocity dependent increase in the tonic stretch
reflex (muscle tone) with exaggerated tendon reflexes, resulting from
the hyper excitability of the stretch reflex, as one component of the
upper motor neuron syndrome" [3]. The exact mechanisms underlying
the development of spasticity are not fully understood [4-5]. Among
individuals with SCI, 65 to 78 percent have symptoms of spasticity [4].

Spinergy wheelchair wheels (Spinergy, Inc; San Diego, California)
are relatively new on the market. These wheels have specialized
features, including a triple-cavity rim, an alloy hub with one-piece
construction, and carbon-fiber spokes that originate from the hub
(reverse spoking). Spinergy claims that as a result of these specialized
features, the wheels absorb 25 percent more road shock than conventional
steel-spoked wheels [6]. If true, this energy absorption would be highly
advantageous in long-term wheelchair use and would suggest that these
wheels could decrease the discomfort caused by WBV. More specifically,
they might reduce spasticity caused by WBV in individuals with SCI. In a
previous study, Hughes et al. compared Spinergy wheelchair wheels with
standard steel-spoke wheelchair wheels in terms of energy expenditure
and user comfort [7]. They found that the Spinergy wheels provided a
more comfortable ride but did not significantly affect energy
expenditure. They suggested that the increased comfort may have
important implications for patient management of pain and spasticity.

The first purpose of this study was to verify Spinergy's claim
that its wheelchair wheels absorb 25 percent more road vibration than
other conventional wheelchair wheel designs. The second purpose was to
assess whether Spinergy wheelchair wheels, as compared with standard
steel-spoked wheelchair wheels, reduce spasticity triggered by wheeling
over rough surfaces and obstacles and improve the comfort level of
individuals with SCI. Our hypothesis was that the Spinergy wheels would
absorb vibration, reduce spasticity triggered by wheeling over rough
surfaces and obstacles, and increase subjective comfort more than the
conventional steel-spoked wheels.

MATERIALS AND METHODS

Part 1: Vibration

The first part of the study addressed the single question of
whether the Spinergy wheels absorb more vibration than conventional
steel-spoke wheels. The experiment consisted of a standardized
coast-down test in which 22 nondisabled subjects rolled down a ramp from
a fixed height in an experimental wheelchair while we evaluated
vibration. We chose the coast-down test for the first part of the study
to provide a method of standardization for velocity, since vibration is
velocity dependent. We chose nondisabled subjects instead of subjects
with SCI since we were not assessing any specific factors related to
SCI. Appropriate university ethics and hospital review certificates were
obtained before data collection.

Subjects

Twenty-two nondisabled subjects participated (12 men, 10 women),
roughly the same number that participated in Hughes et al.'s study
[7]. The mean [+ or -] standard deviation (SD) weight of these subjects
was 71.5 [+ or -] 11.5 kg. They had no previous experience with wheeling
in a wheelchair. After giving informed consent, the subjects started the
experiment. Subjects were randomized to begin with either steel-spoked
or Spinergy wheels.

Wheelchair

All subjects used the same wheelchair, a 15 kg Invacare A4
wheelchair (Elyria, Ohio) that was lent by the GF Strong Rehabilitation
Centre. Tire pressure was kept at 100 psi. The position of the axle
remained constant for all subjects. Steel-spoked wheels were painted
black to look like Spinergy wheels, and Spinergy stickers were removed.
The only obviously visible difference between the two wheel types was
the number of spokes.

Measurement of Vibration

Vibration was measured with two Mechworks MDS 203 two-dimensional
accelerometers (Waterloo, Ontario, Canada). One accelerometer was
mounted on the main axle and the other on the footplate. Both
accelerometers measured accelerations in the fore-and-aft direction (x)
and the vertical direction (y) (Figure 1). The accelerometers were
placed in a fixed position on the wheelchair. The axle accelerometer was
secured with a bolt (Figure 1(b)). The footplate accelerometer was
firmly secured to the best of our capabilities (Figure 1(a)). Horizontal
positioning of the accelerometers was ensured with a level. With this
setup, all acceleration data were in reference to the wheelchair. The
accelerometer has a built-in converter that converts the analog signal
to digital. The accelerometers were directly attached to a laptop. Data
were collected at 1,000 Hz. The footplate was chosen because, according
to Wolf et al. [8], vibration to the limbs can cause musculoskeletal
damage and discomfort. Furthermore, clinical observations suggest that
the initiation of spasticity is due to foot stimulation and a possible
stretch reflex reaction that trigger rapid firing of the gastrocnemius.

[FIGURE 1 OMITTED]

Procedures

In this first part of the study, the subjects sat passively in the
wheelchair and rolled down a ramp with a slope of 8[degrees] after being
released by the researcher. At the bottom of the ramp, the wheelchair
and subject rolled over a small speed bump (0.025 m high x 0.080 m long)
that caused vibration (Figure 2). The accelerometers were started when
the researcher released the wheelchair and stopped when the wheelchair
and subject had rolled over the speed bump. The researcher walked behind
the wheelchair, holding the laptop that collected the accelerometer
data. Since speed affects vibration [2], we examined two different
speeds to validate our measurements. Starting 1.65 and 2.00 m from the
speed bump led to estimated mean speeds at impact of 0.8 and 1.2 m/s,
respectively. These velocities fall within typical wheeling speeds [9].
Each subject performed four test runs: two types of wheelchair wheels at
two different velocities.

[FIGURE 2 OMITTED]

Data Analysis and Statistics

The vibration signals from the accelerometers were analyzed with
MATLAB (version 7.2, The MathWorks, Inc; Natick, Massachusetts). Zero
measurements were subtracted from the acceleration data to eliminate
noise. Peak acceleration and root-mean-square (RMS) values were
calculated in MATLAB. RMS is a measure of the magnitude of vibration and
is the square root of the average of the squares of a set of numbers
(here, the acceleration) [2].

The formula for RMS is stated in the Equation, where x is the
separate data points and N is the number of data points.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

Three types of comparisons were made: (1) wheel type (Spinergy vs
steel-spoked), (2) speed (fast vs slow), and (3) sensor placement (axle
vs footplate). The first comparison addressed the research question,
while the other two comparisons validated the vibration analysis. Two
repeated measures analyses of variance (ANOVAs) were conducted in SPSS
(SPSS, Inc; Chicago, Illinois) for the ramp test: one with the RMS
values and one with the peak accelerations. The three main factors of
the repeated measures ANOVA were wheel type (Spinergy vs steel-spoked),
sensor placement (axle vs footplate), and speed (fast vs slow).

To compare the two wheels in terms of frequency, we obtained a
power spectral density (PSD) analysis from every signal by using fast
Fourier transform analysis. The PSD (range 0-500 Hz) was divided into
bins of 2 Hz each, after which the maximum amplitude within each bin was
taken as a measure of peak power. Subsequently, an ANOVA was conducted
with MATLAB for every 2 Hz bin to compare the peak power between the two
wheels, with speed as the second experimental variable. Significance for
all statistics was set to p < 0.05.

Part 2: Vibration and Spasticity

The second part of the study evaluated whether, compared with
steel-spoked wheels, Spinergy wheels reduce vibration-induced spasticity
in individuals with SCI. This evaluation was made during a test in which
13 subjects with SCI wheeled over nine individual obstacles in their own
wheelchairs but using the two different types of wheels. The vibrations
of the two different wheel types were again compared.

Subjects

Thirteen subjects with SCI participated (10 men, 3 women); their
mean [+ or -] SD age was 46.2 [+ or -] 11.2 years. The aim was to
include 20 persons, similar to the study by Hughes et al. [7]. Table 1
shows the main characteristics of the study participants.

* Independent manual wheelchair use with sufficient strength to
wheel over the obstacles.

* No changes to current wheelchair setup for at least 6 months.

* Ability to understand the instructions and give informed consent.

The exclusion criterion was--

* Any history of cardiovascular disease that would inhibit
performance or make participation unsafe for the subject.

Once we determined that the subjects met the inclusion criteria,
informed consent forms were completed. To understand the subjects'
baseline level of spasticity, a rehabilitation physician completed the
MAS [10].

Appropriate university ethics and hospital review certificates were
obtained before data collection. Subjects were recruited from the
outpatient Spinal Cord Injury Program at the GF Strong Rehabilitation
Centre. Subjects received a modest honorarium for their participation in
the study.

Wheelchairs

Subjects used their own wheelchairs and were provided either a
smooth or plastic-coated handrim on the experimental wheels to ensure
their normal wheeling style. Characteristics of the subjects'
personal wheelchairs are shown in Table 2.

The two wheel types were randomized. The Primo (Primo Wheelchair
Tires, Inc; Philadelphia, Pennsylvania) v-track tires used were inflated
to 100 psi. Subjects were randomized to either start with the Spinergy
or steel-spoked wheels.

Measurement of Vibration

We measured vibration using the same protocol outlined for the
first part of the study.

Measurement of Spasticity and Comfort

Immediately after each trial, the subjects used visual analog
scales (VASs) to answer questions about the severity of their spasticity
and their level of comfort during the trial, as suggested by Platz et
al. [11]. The extremes for the spasticity VAS were "no spasms"
and "worst it could be," and for the comfort VAS,
"extreme discomfort" and "extreme comfort." After
the subjects completed all nine trials, they completed five VASs about
their overall assessment of the wheels (comfort, spasticity, support and
stability, maneuverability, and comfort of hand on pushrim).

Procedures

The obstacles in the test were similar to those used in the
obstacle course previously described and validated by DiGiovine et al.
[12]. The obstacle test consisted of a set of nine obstacles that
resembled as much as possible reallife obstacles that people come across
in their daily lives. We also used this course for our previous study
with the Spinergy wheels (Hughes et al. [7]). In contrast to this
previous study, subjects in the current study wheeled over each obstacle
individually, instead of in one continuous loop, to better control for
velocity, since vibration is velocity dependent [2]. The nine obstacles
are listed in Table 3. The obstacle test setup is shown in Figure 3.

Once the first set of wheels was mounted to the subject's own
wheelchair, the subject was asked to wheel over each obstacle. The
obstacles were placed in the center of a gymnasium. The subject started
behind a line on one end of the gymnasium, wheeled 2.6 m, went over the
obstacle, and continued wheeling until crossing a line at the other end.
This sequence represented one trial. To calculate average velocity, we
used a stopwatch to measure the time the subject took to complete the
trial. This process was repeated for the second set of wheels. The
sequence of the obstacles was also randomized. The subjects had to
complete 18 trials: nine different obstacles with two different types of
wheels.

Data Analysis and Statistics

The VASs on spasticity, comfort, and overall rating of the wheels
and the average trial velocity were analyzed with a paired-samples
t-test in SPSS to compare the two different wheel types at each
obstacle. The vibration signals from the accelerometers were analyzed in
the same way as in the first part of the study, except that the third
factor in the repeated measures ANOVA was obstacle instead of speed. We
used subjects as their own controls by using a within-subject
comparison. Significance for all statistics was set to p < 0.05.

RESULTS

Part 1: Vibration

All 22 nondisabled subjects completed the ramp test. Figure 4 shows
a typical example of the acceleration signal and its power spectrum. All
data are presented as mean [+ or -] standard deviation unless otherwise
indicated.

[FIGURE 3 OMITTED]

Wheel Types

No significant differences were found between the two wheel types
for peak acceleration (Spinergy: 2.84 [+ or -] 1.16 g, steel-spoked:
2.81 [+ or -] 1.09 g), RMS (Spinergy: 0.33 [+ or -] 0.10, steel-spoked:
0.33 [+ or -] 0.10), or peak power. Validation of Vibration Analysis
Over the whole data set (data from the different wheel types combined),
significant differences were found for peak acceleration between the
different positions of the accelerometers (footplate: 3.41 [+ or -] 1.01
g, axle: 2.24 [+ or -] 0.90 g, p < 0.001) and the different speeds
(fast: 3.07 [+ or -] 1.11 g, slow: 2.59 [+ or -] 1.09 g, p < 0.001).
Similar significant differences were found for RMS between the positions
of the accelerometers (footplate: 0.40 [+ or -] 0.09, axle: 0.26 [+ or
-] 0.05, p < 0.001) and the speeds (fast: 0.35 [+ or -] 0.09, slow:
0.31 [+ or -] 0.09, p < 0.001). Peak accelerations and RMS values
were higher at the footplate than at the axle and were higher at the
higher velocity.

[FIGURE 4 OMITTED]

Part 2: Vibration and Spasticity

All subjects completed the obstacle course. One subject did not
feel comfortable wheeling over the ramp; hence, n = 12 for the ramp
(obstacle 4) and n = 13 for the other obstacles. Average speed did not
differ significantly between the two wheel types.

Over the whole data set (data from the different wheel types
combined), significant differences were found between the different
positions of the accelerometers for peak acceleration (footplate: 2.76
[+ or -] 2.39 g, axle: 1.90 [+ or -] 2.03 g, p < 0.001) and RMS
(footplate: 0.40 [+ or -] 0.09, axle: 0.26 [+ or -] 0.05, p < 0.001).
The peak accelerations and RMS values were higher for the footplate.

Spasticity and Comfort

The VAS on spasticity was not significantly different between the
different wheel types for any of the obstacles (Figure 5). The VAS on
comfort also did not significantly differ between the Spinergy and
steel-spoked wheels for any of the obstacles.

Overall Assessment

The VASs on overall assessment of the wheels did not show any
significant differences between the Spinergy and steel-spoked wheels.

DISCUSSION

Vibration For both parts of the study, no significant differences
were found between the Spinergy and steel-spoked wheels in peak
acceleration, RMS, or peak power. For peak power, only a few significant
differences were found between the power bins over the whole frequency
spectrum, but they were not consistent across the conditions.
Significant differences were found between the two speeds and the two
positions of the accelerometers in the first part of the study. The
higher speed led to higher peak accelerations. This result was expected,
since reaching the speed bump at a higher speed would logically result
in higher acceleration peaks. The footplate peak accelerations were
significantly higher than the axle peak accelerations. This result was
also expected, since the mass at the footplate to which the force
(shock) is being applied is significantly lower than at the axle,
resulting in higher peak accelerations. Furthermore, smaller caster size
at the footplate will result in higher accelerations and deformation of
the tires, tubes, and rims, and the spokes on the rear wheels act to
dampen accelerations transmitted in the rear of the wheelchair. The
results for velocity and position of the accelerometer met all
theoretical expectations, thus validating the experimental approach and
technique for the evaluation of vibration exposure.

[FIGURE 5 OMITTED]

For the frequency analysis, grouping the frequency ranges and
assigning them to one of the two wheel types would have been preferable.
Cooper et al. [13] and DiGiovine et al. [14] compared frequency in
wheelchair research by dividing the frequency range into octaves and
subsequently comparing within each octave. The downside of this kind of
analysis is that octaves are of different lengths, which makes
interpreting the results difficult. VanSickle et al. divided the
frequency range into equal bins of 3.125 Hz [15]. Griffin provided
proportional bandwidth analysis (octaves) and constant bandwidth
analysis as options for frequency analysis [2]. For nondisabled people
in a sitting position, 4 to 12 Hz has been determined to be the most
dangerous WBV frequency range [2]. However, no research-based values are
available for people with SCI and spasticity. Therefore, we chose a
constant bandwidth analysis. For the same reason, we did not apply the
frequency weightings specified by the International Organization for
Standardization (ISO) 2631-1 [16] when calculating RMS. These weightings
are based on different sensitivity of the body to vibration in each
axis, something that has not been researched in people with SCI. Another
reason we did not apply the frequency weightings was the placement of
the accelerometers: they were not placed exactly in line with the axes
of the body, as ISO 2631-1 prescribes [16]. Future research should be
directed toward the question of which frequency ranges trigger
spasticity and/or create discomfort or health risks among people with
SCI. Subsequently, future research should focus on developing a
wheelchair that specifically targets those frequencies for vibration
dampening.

In a wheelchair study with a similar obstacle course [14],
accelerations were analyzed by means of a vibration dose value (VDV).
The VDV is a cumulative measure of the vibration absorbed by a person
over a certain time period [2]. The focus of this study was not
cumulative vibration and shocks; thus, the VDV was not useful for our
analysis. VanSickle et al. [15] and DiGiovine et al. [14] used a
bite-bar to measure transmissibility of vibrations onto the body. Since
the current study was focused on vibration exposure on the wheelchair
rather than absorption of vibration in the body, we chose not to measure
vibration transmission. It could be that Spinergy wheelchair wheels
reduce transmissibility of vibrations from the wheelchair onto the body.
This possible effect requires further research with a somewhat
differently designed study and different outcome measures.

We recognized that different speeds might generate different
vibrations, thereby making the results dependent on the rate of
propulsion [14]. As a result, we chose the method used in the first part
of the study to control for velocity. Since subjects served as their own
controls, we believed we could reasonably compare the two types of
wheels without speed being a confounder.

Spinergy claims on its Web site that its fiber spokes act as
vibration and shock dampeners--25 percent more absorbent than steel [6].
It could be that the material itself (PBO fiber) does reduce vibrations
by 25 percent but that this effect cannot be extrapolated to the
vibration characteristics of an entire wheelchair wheel.

Spasticity and Comfort

The spasticity and comfort results are in line with the vibration
results; no differences in vibration exposure were seen between the
wheel types, so an effect on spasticity and/or comfort would not be
expected given the hypothesized relation among these phenomena in SCI.

The VASs showed no significant difference between the wheels on
either spasticity or comfort. The results in the graph (Figure 5)
indicate a trend toward steel-spoked wheels being rated as higher in
terms of spasticity for eight of the nine obstacles (p = 0.06). However,
because of the large variability in the data, this trend did not reach
significance. With a larger sample size, a significant trend might have
been attained. The VAS results on comfort did not confirm the results of
Hughes et al. [7]. In a similar study also comparing Spinergy versus
steel-spoked wheels on energy efficiency, Hughes et al. found Spinergy
wheels to be preferred over steel-spoked wheels in terms of comfort [7].
The difference in the results could be explained by the fact that Hughes
et al. [7] used the obstacle course previously described by DiGiovine et
al. [12], in which the subjects wheeled consecutively over all the
obstacles in one trial. Therefore, subjects had to maneuver the
wheelchair between the obstacles (make turns, brake, accelerate, and
decelerate), unlike in the current study. Spinergy wheels may be more
comfortable in terms of general wheelchair use and maneuverability;
however, we are not able to confirm this hypothesis.

Some subjects had severe visible spasms during the transfers, but
these kinds of spasms were not observed during the wheeling tests. The
obstacle course may not have sufficiently simulated the experiences
individuals have in the community. Also, one must consider that, up to
now, no objective measurements for spasticity were suitable for this
kind of study [17]. The VAS might have failed to detect a difference in
spasticity because of its subjective nature. Wewers and Lowe mention
that the necessary conditions for reliability and validity of the VAS
remain unresolved [18]. Despite the fact that no articles were found in
which the VAS was used as a measure of spasticity, we chose the method
for lack of finding a better one. In a study by Lingjaerde and Foreland
[19], the VAS showed excellent test-retest reliability and high validity
while measuring depression. In their review on clinical scales for the
measurement of spasticity, Platz et al. mentioned that the VAS as a
self-report scale on spasticity might add valuable information [11]. A
better understanding of the syndrome of spasticity and the development
of a valid, reliable assessment tool are needed [4]. In future research,
electromyography (EMG) could provide a more objective measurement
[17,20]. The downside of EMG is that subjects will have to deal with
wires that can obstruct wheeling and functioning.

Some discussion has occurred regarding the reliability and validity
of the MAS [21-22]. Bakheit et al. suggested that the MAS measures
muscle hypertonia rather than spasticity [23]. Furthermore, Blackburn et
al. concluded that when assessing muscle tone, the MAS yields reliable
measurements but only for a single examiner [10]. In our study, the same
physician performed the MAS for every measurement. Since no other
reliable and valid objective assessment tools exist to measure
spasticity [17], the MAS was the best alternative.

Protocol

One change we made from the previous DiGiovine et al. [12] and
Hughes et al. [7] studies was to separate out each obstacle in the
second part of the study (i.e., one trial represented one obstacle
instead of an obstacle course). We included this change to ensure that
the previous obstacle had no influence on the outcome of the next
obstacle and so that we could individually evaluate each obstacle. In
addition, this change in protocol attempted to standardize speed, a
limitation of the setup in DiGiovine et al. [12]. We did not find that
Spinergy wheels, compared with standard steel-spoked wheels, had
beneficial effects with respect to vibration, spasticity, and comfort.
Factors such as weight of the wheels (Spinergy wheels are lighter than
steel-spoked wheels) could make Spinergy wheels preferable.

Limitations

Most of the subjects in our study had fairly wellmanaged
spasticity, which may have limited the effect of the vibrations. Most
subjects used some kind of medication to inhibit their spasticity,
usually baclofen or Lyrica. For ethical reasons, we could not ask them
to stop their medication. Even though the medication does not completely
take away all spasms, it may have affected our results. It would be
interesting to test those subjects who have more difficulties managing
their spasticity and see whether the Spinergy wheels offer more comfort,
as seen in the previous study [7].

Completely controlling for velocity is difficult. In the second
part of the study, speed was not completely standardized like it was in
the first part. We attempted to standardize velocity by adjusting the
protocol of DiGiovine et al. [14]. Instead of wheeling over all the
obstacles at once, subjects wheeled over one obstacle at a time. The
average velocity was calculated per obstacle and was not significantly
different between the wheels. We found that subjects used different
wheeling strategies over the obstacles, especially the big speed bump.
Some did a "wheelie" (wheeling on hind wheels), while others
went over the bump slowly on four wheels. The difference in strategy
could have affected the outcome measures.

In the first part of the study, the nondisabled subjects used one
experimental wheelchair, while in the second part, the subjects with SCI
used their own wheelchairs, causing an extra dimension of variation.
However, since comparisons were made within subjects, this variation was
assumed to not be a confounder.

One aim of this study was to stay close to real-life situations.
The downside of this approach is that several variables could have had a
confounding effect on the results. Such variables include the
subjects' height, weight, and technique while wheeling over the
obstacles. To understand whether specific frequencies trigger spasms, we
need a more standardized approach; this approach might include the
creation of a vibrating plate [24] with variable vibration frequencies
for subjects to sit on, as well as the use of EMG of the leg muscles to
measure the response to the vibration, rather than relying only on
subjective feedback. A study without people sitting in the wheelchair
would enhance standardization of the vibration analysis, for example,
use of a double drum comprised of a little bump [13]. For the
measurement of the effect of wheelchair vibration on spasticity,
standardization would be enhanced if people with SCI were to sit in
their wheelchair on a standardized vibration stimulator.

CONCLUSIONS

We can conclude that under the current standardized conditions, the
Spinergy wheelchair wheels, as compared with the standard steel-spoked
wheelchair wheels, neither absorb more vibration at the footplate or the
axle nor reduce perceived spasticity or improve comfort in individuals
with SCI wheeling over rough surfaces and obstacles.